Actually Grub eggs hatch better in wet soil than dry. The eggs take on over 50% of their weight from water as they mature. It has been my experience to have less grubs in dry years and more Grubs in Wet years.

The trade off is you have more damage in dry years simply because of irrigation problems and lawns that are un or under irrigated have less recuperative capability.

Its kindof the same with surface insects. You always have them but you only notice extensive damage in dry seasons.

A lot of these insecticides break down in sunlight too so letting merit sit there unirrigated for a week can't do it any good.